This invention relates to pulsed lasers and to methods for generating pulsed laser radiation and, more particularly, to passively. mode-locked optically pumped semiconductor external-cavity surface-emitting lasers (OPS-EXSELs).
Semiconductor lasers are known in the art. Their laser gain medium consists of a semiconductor material such as InGaAs. In most cases, they do not require any external resonator because the end faces of the semiconductor material can be designed as the resonator mirrors. They can be pumped electrically by applying an appropriate voltage to the semiconductor material. The so-called bandgap engineering, a technique making use of the large number of known semiconductor materials and laser designs, offers a great variety of emittable wavelengths in the infrared and visible range. Semiconductor lasers are small and compact, and can be manufactured in great masses at low costs.
Semiconductor lasers can be designed either as edge-emitting lasers or as surface-emitting lasers. Edge-emitting lasers are the most common form of semiconductor lasers, but this concept very much limits the mode area in the device. For ultrashort pulse generation, a consequence of this is that the pulse energy is also limited to values far below what is achievable, e.g., with lasers based on ion-doped crystals. Also a high average output power (a few watts or more) cannot be generated with good transverse beam quality. These problems can be solved with surface-emitting semiconductor lasers, where the mode areas can be greatly increased, particularly if the device is optically pumped.
Electrically pumped vertical-cavity surface-emitting lasers (VCSELs) known to date are limited in their output power or in terms of beam quality. That is because in a small-area VCSEL the heat dissipation limits the driving current, while in a large-area VCSEL the pump distribution is not uniform enough to support fundamental-transversal-mode operation. With optical pumping the problem of the pump uniformity can be overcome and an external cavity ensures stable fundamental mode operation even with a large mode size. (M. Kuznetsov et al., xe2x80x9cHigh-power ( greater than 0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM00 beamsxe2x80x9d, IEEE Phot. Tech. Lett.,Vol. 9, No. 8, p. 1063, 1997) The extensive gain bandwidth of semiconductor quantum well lasers is attractive for ultrashort pulse generation. Lasers emitting short (in the nanosecond and sub-nanosecond range) or ultrashort (in the sub-picosecond range) pulses are known in the art. A well-known technique for short or ultrashort pulse generation is mode locking. Mode locking is a coherent superposition of longitudinal laser-cavity modes. It is forced by a temporal loss modulation which reduces the intracavity losses for a pulse within each cavity-roundtrip time. This results in an open net gain window, in which pulses only experience gain if they pass the modulator at a given time. The loss modulation can be formed either actively or passively. Active mode locking is achieved, for instance, using an acousto-optic modulator as an intracavity element, which is synchronized to the cavity-roundtrip time. Active mode locking of a diode-pumped quantum well laser has, e.g., been achieved with an intra-cavity acousto-optic prism, giving pulse lengths of 100-120 ps (M. A. Holm, P: Cusumano, D. Burns, A. I. Ferguson and M. D. Dawson, CLEO ""99 Technical Digest, Baltimore 1999, paper CTuK63).
However, ultra-short-pulse generation relies on passive mode-locking techniques, because only a passive shutter is fast enough to shape and stabilize ultrashort pulses. Passive mode locking relies on a saturable absorber mechanism, which. produces decreasing loss with increasing optical intensity. When the saturable-absorber parameters are correctly adjusted for the laser system, stable and self-starting mode locking is obtained. Saturable-absorber mode locking of diode lasers has been widely investigated, originally using a semiconductor saturable absorber mirror (SESAM) in an external cavity (Y. Silberberg, P. W. Smith, D. J. Eilenberger, D. A. B. Miller, A. C. Gossard and W. Woiegan, Opt. Lett. 9, 507, 1984), and more recently in monolithic devices, which use sections of reverse-biased junction to provide saturable absorption (for a review, see xe2x80x9cUltrafast Diode Lasers: Fundamentals and Applicationsxe2x80x9d, edited by P. Vasilev, Artech House, Boston, 1995). A harmonically mode-locked monolithic laser was shown to generate picosecond pulses at a repetition rate variable up to 1.54 THz (S. Arahira, Y. Matsui and. Y. Ogawa, IEEE J. Quantum Electron. 32, 1211, 1996); however such devices are limited to a few tens of milliwatts of output power.
Another approach for short-pulse generation was to use a mode-locked dye or solid-state laser as a synchronous optical pump for a vertical-external-cavity surface-emitting laser (VECSEL) (W. B. Jiang, R. Mirin and J. E. Bowers, Appl. Phys. Lett. 60, 677, 1992). These lasers typically produced chirped pulses with a length of about 20 ps, which were externally compressed to sub-picosecond, and even sub-100-femtosecond duration (W. H. Xiang, S. R. Friberg, K. Watanabe, S. Machida, Y. Sakai, H. Iwamura and Y. Yamamoto, Appl. Phys. Lett. 59, 2076, 1991). The general drawback of this approach, which prevents widespread applications, is that the pumping laser itself has to deliver ultrashort pulses. This severely limits the attractiveness of the overall system in terms of complexity, size, cost, and achievable pulse repetition rate.
In U.S. Pat. No. 5,461,637 (Mooradian et al.), a vertical-cavity surface-emitting laser (VCSEL) is disclosed with a quantum-well region formed over a semiconductor substrate. A first reflective surface is formed over the quantum-well region, and a second reflective surface is formed over the substrate, opposite the first reflective surface, forming a laser cavity. However, there is no teaching about measures to be taken for mode locking such a VCSEL.
It is an object of this invention to provide a simple, robust laser emitting short (in the picosecond range) or ultrashort (in the sub-picosecond range) pulses, with a high repetition rate (in the range of a few GHz or higher), with a high optical average output power (of at least hundreds of milliwatts) and a good beam quality (coefficient of beam quality M2xe2x89xa65; cf. T. F. Johnston, Jr., xe2x80x9cM2 concept characterizes beam qualityxe2x80x9d, Laser Focus World, May 1990).
It has been found that the combination of an optically pumped external-cavity surface-emitting laser (EXSEL) with a semiconductor saturable absorber structure solves the above problem. Thus the laser according to the invention comprises a surface-emitting semiconductor laser with an external cavity. The laser is pumped optically, preferably with a high-power diode laser bar. Finally, it is passively mode-locked with a SESAM in the external cavity, or alternatively with a saturable absorber which is incorporated into the semiconductor laser structure. SESAM stands here for any semiconductor saturable absorber structures, which have sometimes been termed A-FPSA (Opt. Lett. 17, 505, 1992), SBR (Opt. Lett. 20, 1406, 1995), D-SAM (Opt. Lett. 21, 486, 1996), semiconductor doped dielectric layers (Opt. Lett. 23, 1766, 1998), or colored glass filters (Appl. Phys. Lett. 57, 229, (1990), for example. Any other saturable absorbers could be used which allow to adjust the operation parameters for stable mode locking (cf. C. Hxc3x6nninger et al., xe2x80x9cQ-switching stability limits of cw passive mode lockingxe2x80x9d, J. Opt. Soc. Am. B 16, 46, 1999).
More particularly, the laser according to the invention comprises:
a first reflective element and a second reflective element being separated therefrom, said first and second reflective elements defining an optical resonator for laser radiation;
an essentially plane semiconductor gain structure having a surface extending essentially in a surface plane, for emitting said laser radiation;
means for exciting said semiconductor gain structure to emit said laser radiation from said surface plane, said exciting means comprising a pumping source for emitting pumping radiation which impinges on said semiconductor gain structure; and
a semiconductor saturable absorber structure for mode locking said laser radiation.
The method for generating pulsed electromagnetic laser radiation according to the invention comprises the steps of:
generating pumping radiation;
exciting an essentially plane semiconductor gain structure, which has a surface extending essentially in a surface plane, to emit laser radiation from said surface, by impinging said pumping radiation on said semiconductor gain structure;
recirculating said laser radiation in an optical resonator; and
mode locking said laser radiation by means of a semiconductor saturable absorber structure.
In the following we explain how this invention solves a number of problems which are related to previously used approaches. By using a semiconductor gain material, a broad amplification bandwidth is obtained as required for the generation of ultrashort pulses. The relatively small saturation energy of the semiconductor gain medium is beneficial for pulse generation at high repetition rates, as explained below. The surface-emitting geometry allows for a relatively large laser-mode area which reduces the optical peak intensities on the semiconductor and thus allows for large pulse energies. For operation with multi-watt output powers, electrical pumping of the gain medium is not a good option because in this way it is difficult to obtain a sufficiently uniform pumping density over a large mode area. Optical pumping eliminates this problem and at the same time gives more design freedom for the optimization of the gain structure. A high-power diode bar is most suitable as a pumping source, being compact and delivering tens of watts of pumping light with good efficiency, while the poor beam quality is not important due to the very small absorption length of the gain structure. Furthermore, the external laser cavity determines the laser repetition rate (via the cavity length) and also allows to incorporate a SESAM. The latter (or alternatively, a saturable absorber incorporated into the gain structure) leads to mode locking, i.e., the formation of short or ultrashort pulses with a spacing according to the laser cavity length.
The relatively small saturation energy of the semiconductor gain medium is very important for pulse generation at high repetition rates. Other passively mode-locked lasers, based on ion-doped crystals, have a much larger gain saturation energy. (This is particularly the case for most ion-doped gain materials with broad amplification bandwidth, as required for sub-picosecond pulse generation.) For this reason, such lasers have a tendency for Q-switching instabilities (or Q-switched mode locking, QML, see C. Hxc3x6nninger et al, J. Opt. Soc. Am. B 16, 46, 1999). This tendency is very difficult to suppress if a high pulse repetition rate is required, and particularly if a high output power is required at the same time. Because of their much smaller gain saturation energy, semiconductor lasers substantially do not exhibit these problems and therefore are suitable for the generation of pulse trains with high repetition rates and high average powers.
An important design criterion is that the saturation energy of the laser gain structure must be larger than the saturation energy of the saturable absorber. For stable mode locking, the ratio of these two quantities preferably should be 2 or even larger. If a SESAM is used for passive mode locking, its saturation energy can be adjusted both by the SESAM design and by the mode area on the SESAM, the latter being controlled by the laser cavity design. Typically, the mode area on the SESAM would be significantly smaller (e.g., more than five times smaller and preferably more than ten times smaller) than the mode size on the gain structure. For a saturable absorber which is incorporated in the gain structure, a suitable ratio of saturation energy can be obtained through a proper design. In particular, the device can be designed so that the optical intensities in the absorber structure are larger than the intensities in the gain structure, e.g., by exploiting the spatially varying intensities due to a standing-wave field in the structure or by coupled cavities, where in one cavity is the gain and in the other the absorber. For broad-band operation, the coupled cavity should be at antiresonance. Alternatively, the intrinsic saturation energies of absorber and gain structure can be controlled by band gap engineering.
For ultrashort pulse generation, the design of the laser gain structure must avoid bandwidth-limiting coupled-cavity effects which can arise from internal reflections, e.g., from the surface of the laser gain structure. Such reflections effectively modulate the gain spectrum of the device, which limits the usable gain bandwidth. One possibility is to suppress such reflections by arranging semiconductor layers (or possibly layers made from other materials such as dielectrics) so that the reflections from the single interfaces effectively cancel out. (This is basically the principle of anti-reflection coatings.) Another possibility is to allow for some reflection from the surface of the gain structure, but design the thickness of the whole structure so that it is anti-resonant over the whole wavelength range where there is gain. This somewhat increases the pump threshold of the device, but it also increases the effective gain saturation energy which can be beneficial as explained above.
Reflections from the back side of the semiconductor substrate (on which the gain structure is grown) can also significantly affect the device performance, even if the residual transmission of the Bragg-mirror structure is quite small. The reason for this is that Fabry-Perot effects can arise from the reflections of Bragg mirror and the back side of the substrate. Increasing the reflectivity of the Bragg mirror reduces such effects, but roughening or angle polishing of the substrate is a simple and effective alternative.
The device is preferably operated with a single pulse circulating in the cavity. However, harmonic mode locking may be used to achieve a higher repetition rate.
This means that several pulses are circulating in the laser cavity with a fixed spacing. This regime of operation can be realized, e.g., by addition of a suitable-spectral filter in the laser cavity, or by placing the saturable absorber at a place in the cavity where counterpropagating pulses meet.
It should be noted that the laser concept according to the invention is power scalable. For example, doubling of the output power is possible by using twice the pumping power, while the mode areas on the gain structure and the saturable absorber structure are doubled at the same time. Gain and absorber structure are then operated with the same intensities as in the original device. The temperature rise on both structures is also not significantly increased because the mode diameter can be made larger than the thickness of the gain structure and the absorber structure; for this case, simulations have shown that the heat flows essentially in one dimension, i.e., in the direction in which the thicknesses of the structures are measured. Also the mode-locking performance is not affected due to the unchanged intensities. This scalability is not given for edge-emitting lasers, nor for electrically pumped surface-emitting lasers.
EXSELs (and also VECSELs) use a semiconductor wafer in which one quantum well or a stack of quantum wells is grown adjacent to a single Bragg-mirror structure or a metallic mirror. It is also possible to consider a thicker bulk layer for the gain medium. However, it is expected that quantum-well gain layers are better for the laser threshold, but for mode locking it could be useful that the saturation energy for bulk is higher than in a quantum well. Light from one or more multi-mode high-power diode lasers is focused onto the face of the wafer and pumps the wells by absorption in the barrier regions. The area of the laser mode on the active mirror can be about 104 times larger than the modearea on the facet of an edge-emitting laser, offering scope for the generation of high average power and large pulse energy. At the same time the external cavity enforces fundamental mode operation in a circular, near-diffraction-limited beam.
With the laser according to the invention, sub-picosecond pulse durations are achievable by eliminating coupled cavity effects and by external pulse compression. Such devices are substantially free of the Q-switching tendency that is inherent in passively mode-locked dielectric laser systems with high repetition rates (cf. C. Hxc3x6nninger, R. Paschotta, F. Morier-Genoud, M. Moser and U. Keller, J. Opt. Soc. Am. B, 16, 46, 1999). Band-gap and device-structure engineering allows to cover a large wavelength region with the same laser technique, and it even allows to shape the pulses or to integrate gain and saturable absorber within the same wafer. Thus the invention makes possible rugged, efficient pulsed laser sources with high average power in a diffraction-limited beam, sub-picosecond pulse durations, and multi-GHz repetition rates.